1. Field of the Invention
The present invention relates to an apparatus and a method for determining an aperture illumination of a phased-array antenna which includes a plurality of radiating elements coupled via coupling apertures to at least one integral monitor waveguide and wherein a signal-conditioning circuit is connected to a first output of the integral monitor waveguide for determining at least one real part and any existing imaginary parts, of a time-dependent complex monitor signal provided by the integral monitor waveguide, the signal-conditioning circuit feeding the at least one real part and the any existing imaginary parts of the complex monitor signal to a signal processing circuit having a signal processor therein for continuously calculating the aperture illumination of the phased-array antenna from the real and imaginary parts of the complex monitor signal determined by the signal-conditioning circuit.
2. Description of the Prior Art
The apparatus described above, is known from both U.S. Pat. Nos. 5,187,486 and 4,926,186, the entire contents of which are incorporated herein by reference. This known apparatus is used, for example, to monitor phased-array antennas in microwave landing systems (MLS systems).
In MLS systems it is important for safety reasons to constantly monitor the correct operation of the transmitting devices, particularly the functioning of the individual radiating elements of the phased-array antenna. In older MLS systems, this is done, for example, by monitoring currents which flow through PIN diodes connected as phase shifters ahead of the individual radiating elements.
In the apparatus described in U.S. Pat. Nos. 4,926,586 and 5,187,486 mentioned above, the distribution of the far-field of the phased-array antenna is monitored in addition to the diode current. Since the far-field is linked with the aperture illumination of the antenna via a Fourier transform, far-field monitoring makes it possible to detect deviations in both the aperture phase illumination and the aperture amplitude illumination of the individual radiating elements.
In addition to direct far-field measurements, the distribution of the far-field of a phased-array antenna can be determined by means of a so-called integral monitor waveguide, a waveguide component which is arranged parallel to the array axis in the vicinity of the radiating elements and is coupled with the radiation fields of the individual radiating elements via coupling apertures. In such an integral monitor waveguide, the field components from the individual radiating elements are combined to form a time-dependent complex monitor signal which can be obtained as an output of the integral monitor waveguide and whose waveform, if the scan angle of the antenna beam is sufficiently large, corresponds, to a good approximation, of the far-field pattern except for an angular displacement with respect to the normal that is perpendicular to the array axis, i.e., this is the so-called monitor angle. The complex monitor signal has a real part and may have one or more imaginary parts (see U.S. Pat. No. 5,187,486, column 3, lines 6-43 and U.S. Pat. No. 4,926,186, column 8, lines 4-8 and 46-53 and column 9, lines 12-29).
The monitor angle, by which the monitor signal is shifted with respect to the normal to the array axis, can be influenced within certain limits by the dimensions of the integral monitor waveguide and by the shape of the coupling apertures. The monitor angle can be taken into account in calculating the aperture illumination of the antenna, so that this calculation, despite the displacement of the monitor signal by the monitor angle, can be made from this monitor signal by way of a Fourier transform.
A prerequisite for a good match between the monitor signal derived from the integral monitor waveguide and the far-field pattern of the antenna, and thus for a correct calculation of the aperture illumination of the antenna, requires that the antenna be scanned through a sufficiently large angular range. This angular range should cover at least one full cycle of the far-field pattern, so that field information of one complete cycle of the far-field pattern is available for performing the Fourier transform.
In most cases, however, MLS antennas have a restricted scan angle which frequently covers only a fraction of one cycle of the far-field pattern. In such cases, the Fourier transformation of the monitor signal becomes erroneous and, thus, unsuitable. Correction of errors due to use of too small a scan angle can be performed by use of a window function as proposed in the above-mentioned U.S. Pat. No. 4,926,186 in column 9, lines 34-42, which, however, provides no fundamental remedy for the problem of use of too small a scan angle. The use of a window function may possibly only be useful if the scan angle is very much less than one cycle of the far-field pattern.